As structures such as bridges built in Japan's high-growth period have deteriorated, problems have begun to surface. If an accident happens in such a structure, the damage due to the accident is incalculable. Techniques, thus, have been known that monitor the conditions of the structures. For example, an acoustic emission (AE) technique has been known that detects a damage of such a structure using a highly sensitive sensor that detects an elastic wave generated as a result of internal crack occurrence or internal crack growth.
The acoustic emission is radiation of elastic waves generated with fatigue crack growth in a material. In the AE technique, the elastic waves are detected, as a voltage signal (AE signal), by an AE sensor using a piezoelectric element. The AE signal is detected as a sign before material fracture. The occurrence frequency and the signal intensity of the AE signal are, thus, useful indicators that represent the soundness of a material. Therefore, research has been actively conducted on techniques for detecting signs of the deterioration of the structures using the AE technique. Particularly, for corrosion diagnosis of petroleum tanks and in manufacturing processes of machinery, the detection techniques using the AE technique are widely used mainly in Europe and the United States. The detection techniques using the AE technique have been standardized.